kconfig: announce removal of 'kvmconfig' and 'xenconfig' shorthands

kvmconfig' is a shorthand for kvm_guest.config to save 7 character typing.

xenconfig' is a shorthand for xen.config to save 1 character typing.

There is nothing more than that.

There are more files in kernel/configs/, so it is not maintainable
to wire-up every config fragment to the Kconfig Makefile. Hence,
we should not do this at all.

These will be removed after Linux 5.10. Meanwhile, the following
warning message will be displayed if they are used.

WARNING: 'make kvmconfig' will be removed after Linux 5.10
         Please use 'make kvm_guest.config' instead.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>